The Effects of
Underestimating Background Levels
of Ozone on Predicted Vegetation Losses

A very important issue for vegetation is
the definition of natural background
for ozone. This is important because the ozone exposures used
in some experimental control chambers in both the United States
and Europe for vegetation research experiments were much below
the ozone levels occurring in today's environment. The resulting
effect of applying ozone concentrations much lower than observed
at the cleanest sites in the world probably result in overestimating
vegetation growth losses using current models. Theoretical values
have been estimated to be 0.025 ppm. However, hourly average
concentrations of ozone exceed 0.025 ppm even at the most isolated
sites (e.g., South Pole, North Pole, Spitzbergen Norway). Thus,
we are left with the dilemma of deciding whether to accept at
face value a hypothetical 0.025 ppm value or accept the fact
that natural background is actually a range of hourly average
concentrations that are affected by natural processes characteristic
of the local area. Altshuller and Lefohn (1996) discussed ozone
background levels at coastal and inland sites in the United States
and Canada. See our publications list
for more information on background ozone. The ranges suggest
that the background ozone is somewhat dependent on a number of
conditions, such as the nature of upwind flow, lack of pollution
sources, and terrain conditions and natural emissions associated
with forest and agricultural areas. At some locations, stratospheric
ozone plays an important role in determining surface ozone concentrations.